738 ARTHUR C. TROWBRIDGE 



and there is some doubt as to the correctness of tentative conclu- 

 sions; in still other cases, the solution is entirely hypothetical. 

 In some cases various lines of explanations may have partial applica- 

 tion to the observed features. These problems are here discussed 

 individually. 



MANNER OF FORMATION OF THE FANS AND BAJADA 

 CAUSES OF DEPOSITION 



Material was deposited at the foot of the Sierra and Inyo moun- 

 tains, primarily because of an abrupt decrease in the gradient of the 

 streams. When the mountains were first uplifted, and precipitation 

 fell on the slopes, streams formed and flowed swiftly down the 

 sides, carrying material from their channels. When the base of the 

 mountains was reached, the carrying power was suddenly and 

 greatly decreased, and the first deposition resulted. 



Once started, other factors tended to increase the process of 

 deposition. Streams lost volume by sinking into loose material. 

 This not only reduced the volume of the transporting agent, but 

 also lessened the velocity of the water remaining at the surface; 

 both these changes caused deposition. 



The average relative humidity of the Sierra Nevada Mountains 

 is not far from 60 per cent, and that of Owens Valley probably not 

 more than 40 per cent. 1 When the streams reach the plain, evapo- 

 ration is increased; hence loss of volume, loss of velocity, and 

 decrease in carrying power. This would not be so important in the 

 case of the Inyo Mountains, because the difference in humidity 

 between mountains and plains is not so great there. However 

 what little rain falls, is in the mountains rather than on the plains, 

 and evaporation takes place more rapidly in the latter locality. 



Water taken from streams by irrigation so decreases their vol- 

 ume in some other localities as to aid in causing deposition, but such 

 is not the case in this region. The Sierra bajada has been under- 

 going dissection rather than gaining by deposition since man began 

 to irrigate the lands, and the streams on the Inyo fans, running 

 only after rains, are not used for irrigating purposes. 



1 For details of precipitation and evaporation in the valley, see Water Supply and 

 Irrigation Paper, U.S. Geol. Sum., No. 181, pp. 17-25. 



